This first transformation simplifies the symmetry properties of the problem. However, the inductive reductions provided by the computation of the residues
are still quite intricate. The idea is then to proceed to yet another change of unknown function, this time by means of a more involved transform. The latter will then lead to structurally much
simpler, and thus easier to solve, equations satisfied by the new unknown function. As already mentioned, the dawn of this approach goes back to
[1]}, [2]} and it was put in the present form in
[3]}, [4]}, [5]}.
In particular, we refer to [4]}
for the proof.
